The present invention relates, in general, to the field of circulating fluidized bed (CFB) boilers and, in particular, to an improved impact type particle separator which is cooled by inner cooling tubes.
In the CFB power plant field, the separation of solids which are entrained within the flue gas, and the recycle system for returning these solids to a bed of the CFB boiler, are critical to the efficient operation of the boiler. The separation and recycle system has a great impact on the capital and operating costs of the CFB boiler and its ability to follow variations in loads.
Presently known systems for separating the solids from the flue gas in a CFB boiler include an impact type particle separator system. An impact type particle separator system uses an impact primary particle separator in conjunction with a furnace which channels solids/gas flow to the impact type particle separator. Solids are collected by the impact type particle separator and recycled to the furnace.
Often, the separators have collecting elements made of one or more plates shaped and placed in staggered arrays to present a path which may be navigated by the gas stream, but not the entrained particles. One CFB boiler arrangement uses a plurality of impact type particle separators (or concave impingement members or U-beams) at the furnace exit to separate particles from the flue gas. While these separators can have a variety of configurations, they are commonly referred to as "U-beams" because they most often have a U-shaped configuration in cross-section. When applied to a CFB boiler, a plurality of such impact type particle separators are supported within the furnace enclosure and extend vertically in at least two rows across the furnace exit opening, with collected particles falling unobstructed and unchanneled underneath the collecting members along the rear enclosure wall. The gap between each adjacent pair of U-beams in one row is aligned with a U-beam in a preceding or following row of U-beams to present a tortuous path for the flue gas/solids to navigate. The U-beams in each row collect and remove particles from the flow of flue gas/solids, while the flue gas stream continues to flow around and through the U-beam array.
U.S. Pat. No. 4,992,085 to Belin et al. discloses an internal impact type particle separator employing a plurality of U-shaped impingement members organized in at least two staggered rows and positioned in the stream of a flue gas for entraining solid particles.
U.S. Pat. No. 5,025,755 to Eickvonder et al. discloses a labyrinth separator having staggered beams having essentially a U-shaped cross-section configuration. The labyrinth separator is located in a top region of the fluidized bed reactor.
These types of collection elements are generally relatively long in comparison to their width and depth. The shape of the collection elements is usually dictated by two considerations: namely, the collection efficiency of the U-beams themselves and the ability of the U-beams to be self-supporting. When these elements are used, they are generally placed at the furnace exit and not cooled. Their placement at the furnace outlet is to protect the downstream heating surfaces from erosion by solid particles. Thus, the U-beams are exposed to the high temperatures of the flowing stream of flue gas/solids, and the materials used for the U-beams must be sufficiently temperature resistant to provide adequate support and resistance to damage.
Long, self-supporting stainless steel plate channels have been successfully used in CFB boilers for the primary solids collector. The use of higher furnace temperatures would be desirable but limitations of the long term "creep" strength of the commercially available and suitable alloys makes the use of higher temperatures less economical since increased thickness (and, therefore, increased weight) of the plate used in the collection elements is required. The associated increases in the weight of the thicker, longer channel must be carried by the plate making up the channel, as well as by the overall U-beam supports. By breaking up the long collection channel into short segments, the required strength of each short segment is much less than for the long channel due to the series of intermittent supports and the small amount of weight of any individual segment or element.
Methods of making collection elements which are cooled or supported off a cooled structure have usually included collection plates welded to water cooled support tubes. See U.S. Pat. Nos. 5,378,253 and 5,435,820 to Daum et al. However, welding to the cooling tubes increases the opportunity for tube leaks to occur at the welds.
In addition, under this known design structure, the collection element is cooled asymmetrically due to the proximity of the cooled tube or tubes to only some portion of the shaped collection channel segment or element. Thus, the plate forming the collection elements will tend to distort due to the differential expansion of the cooler areas in comparison to the hotter portions of the collection elements.
In addition, it is necessary to protect the tubes themselves from erosion caused by the impacting solids entrained within the solid/gas flow. This protection requires the use of tube shields made of stainless steel, ceramic or other heat resistant alloy or material which must be used along the entire height of the collector, which adds further cost.